1. Field of the Invention
The present invention relates generally to an apparatus for detecting or deciding occurrence of a misfire in an internal combustion engine on the basis of an ion current or the like quantity indicative of a combustion state taking place in the engine. More particularly, the present invention is concerned with a misfire detecting or deciding apparatus for an internal combustion engine which is capable of detecting or deciding occurrence of misfire in the engine with high reliability without resorting to the aid of any complicated logic.
2. Description of the Related Art
In general, in an internal combustion engine (hereinafter referred to simply as an engine) having a crankshaft driven by a plurality of engine cylinders and a camshaft operatively connected with the crankshaft, a reference position signal generated in synchronism with the rotation of the engine is utilized for determining or controllably setting a variety of timings for the engine operation controls such as an ignition timing, a fuel injection timing and the like. To this end, an angular position detector for generating a reference position signal is mounted on the crankshaft or the camshaft at such a position that the reference position signal as generated indicates a predetermined reference position which corresponds to a predetermined crank angle (i.e., angle of rotation of the crankshaft).
When a misfire occurs within an engine cylinder in the ignition cycle as a result of the absence of combustion, abnormal explosion known as after-burning will take place in succession to the ignition cycle, involving injury of the engine cylinder and/or damage of a catalyst employed for catalytic treatment of the exhaust gas due to the contact with an uncombusted gas mixture (i.e., air/fuel mixture undergone no combustion). Under the circumstances, a variety of measures are developed and adopted in an attempt for suppressing occurrence of misfire in the engine, to thereby secure more positive protection of the engine.
For better understanding of the background of the present invention, a misfire detection or decision apparatus for the engine known heretofore will be described in some detail by reference to the drawings.
FIG. 7 is a functional block diagram showing a general arrangement of an engine misfire detection/decision apparatus known heretofore.
Referring to the figure, a reference numeral 1 denotes an angular position detector which is usually constituted by a rotatable toothed disk mounted on a camshaft of an engine and a sensor installed in opposition to the disk for generating a pulse-like reference position signal T.theta. at a reference position corresponding to a predetermined crank angle in synchronism with the revolution of the engine. Usually, the reference position is set at B75.degree. (i.e., at a position 75.degree. before the top dead center in each engine cylinder) or B5.degree.. A reference numeral 2 designates collectively a set of sensors for detecting a variety of engine operation states D such as an intake air flow (or an opening degree of a throttle valve) indicative of an engine load, a rotation speed (rpm) of the engine, an intake air temperature and so forth. A numeral 20 denotes an ion current detector for detecting an ion current I generated within the engine cylinder immediately after the combustion. In other words, the ion current detector 20 serves for a combustion state detecting function for detecting the combustion state in an associated one of the engine cylinders. Of course, such ion current detector 20 may be provided in association with all the engine cylinders or alternatively for a given number of the engine cylinders, respectively, as occasion requires. A reference numeral 3 generally denotes a control unit which is usually constituted by a microcomputer and which includes an engine control parameter setting means 31 for arithmetically determining a control parameter Ta for each engine cylinder on the basis of the reference position signal T.theta. and the engine operation state signals D mentioned above and a misfire detecting means 32 for detecting the misfire event and generating a misfire detection signal C on the basis of the reference position signal T.theta. and the detected ion current value (indicative of the combustion state) I.
The engine control parameter setting means 31 is designed for generating as the engine control parameter Ta a control timing signal which corresponds, for example, to the ignition timing and at the same time performing a misfire suppression processing (e.g., control of the refiring for the engine cylinder in which the misfire took place) on the basis of the misfire detection signal C generated when the detected ion current value I indicates a misfire level. As the engine control parameter Ta, not only the ignition timing but also other various parameters such as the fuel injection timing, the ignition coil on/off timing, etc., can be employed.
FIG. 8 is a circuit diagram showing a structure of the ion current detector 20. As can be seen from this figure, the ion current detector 20 is composed of an ignition coil 21 having a primary winding 21a and a secondary winding 21b, a power transistor 22 for breaking a primary current i.sub.1 flowing through the primary winding 21a in response to an ignition trigger pulse P generated in an ignition timing sequence, a spark plug 23 for producing a spark through electric discharge brought about by a high voltage induced in the secondary winding 21b upon turning-off of the primary current, a DC power supply source 24 for deriving as an ion current i those ions which are produced by the explosive combustion primed by the spark discharge in the spark plug 23, a resistor 25 connected in series to the DC power supply source 24 for converting the ion current i into a voltage signal and an output terminal 26 for outputting the detected ion current I in the form of the voltage signal mentioned above.
FIG. 9 is a waveform diagram showing a waveform of the ion current i. As can be seen from this figure, the ion current (of negative or minus polarity) i assumes a maximum level in the vicinity of the crank angle of A10.degree. (10.degree. after the top dead center) in succession to the explosion triggered by the spark discharge produced at the ignition plug 23 upon turning-off of the primary current i.sub.1 in response to the ignition trigger or control pulse P.
Now, description will turn to the operation of the engine misfire detection/decision apparatus shown in FIG. 7 by reference to FIGS. 8 and 9.
Usually, the engine control parameter setting means 31 sets the ignition timing (i.e. the time point for ignition) with reference to the reference position which corresponds to a rising edge or a falling edge of the reference position signal T.theta. and determines the ignition timing so as to be optimal for the prevailing engine operation state D by consulting a data map or table, to thereby output as the control parameter Ta a gap time or duration which is to intervene between the reference position and the ignition time point.
On the other hand, the misfire detecting means 32 determines the combustion state within the engine cylinder in each ignition cycle on the basis of the reference position signal T.theta. output from the angular position detector 1 and the detected ion current value I output from the ion current detecting circuit 20 and generates the misfire detection signal C for the engine cylinder for which the detected ion current value I produced immediately after the explosion stroke is lower than a predetermined reference level. The engine control parameter setting means 31 responds to the misfire detection signal C input thereto by correcting the control parameter Ta for the engine cylinder misfired so that occurrence of misfire in that cylinder can be suppressed. To this end, the ignition timing control can be modified appropriately or alternatively the ignition energy may be increased by elongating the electrical conduction of the primary current i.sub.1 through the ignition coil 21. Further, in association with the fuel injection control, the injection period may be increased to enrich the air/fuel mixture. In case the misfire susceptibility is not improved even by the correction of the control parameter Ta as mentioned above, fuel injection to the engine cylinder suffering the misfire can be stopped to thereby prevent the discharge of the uncombusted gas to the atmosphere.
In general, when the power transistor 22 is turned off in response to the ignition control pulse P in the ignition cycle, a high voltage of negative polarity is applied across the spark plug 23 connected to the secondary winding 21b of the ignition coil 21, as a result of which an electric discharge takes place between a pair of electrodes of the spark plug 23 to fire the gas mixture which then undergoes an explosive combustion. At this time, ions are produced within the engine cylinder due to ionization brought about by the explosive combustion. After the explosion, the electrode of the spark plug 23 to which a bias voltage is applied from the DC power supply source 24 serves as an electrode for detecting the ion current i.
The ions inclusive of electrons produced within the engine cylinder are caused to migrate under the effect of the electric field of the bias voltage of positive (plus) polarity supplied from the DC power supply source 24, giving rise to the ion current i, which is then converted to the detection voltage I by the resistor 25 to be outputted from the output terminal 26. Thus, it is possible to make a decision as to whether or not the combustion has taken place in the engine cylinder in the ignition cycle by checking the level of the detected ion current signal I.
However, since the misfire phenomenon may also occur in dependence on the engine operation or running states D and other conditions regardless of the actual states of the engine cylinder, it is not proper to make a decision as to the occurrence of misfire only on the basis of the misfire detection signal C. Further, the control procedure for coping with the misfire should preferably be modified by taking into consideration a misfire rate which represents susceptibility to the misfire or likelihood of occurrence of misfire.
The misfire rate may be determined on the basis of the number of the misfire detection signals C generated during a predetermined period. However, it is impossible to determine the misfire rate with a reasonable accuracy unless the timing at which the misfire detection signal C is generated is taken into account, which however requires a complicated implementation of decision logic. In reality, any approach proposed heretofore for determining the misfire rate has lead to very complicated logical configuration of the misfire detecting means 32.